The effects of utterance timing and stimulation of left prefrontal cortex on the production of referential expressions

•We test how the timing of utterance initiation affects the usage of pronouns and zeros.•We use tDCS to stimulate left PFC to understand how it modulates utterance planning and reference.•Message pre-planning increases the use of pronouns/zeros and other markers of discourse coherence.•Temporal breaks in the visual input create discourse breaks and discourage reduced forms.•Stimulation of left PFC eliminates the deleterious effects of incremental planning.

We examined the relationship between the timing of utterance initiation and the choice of referring expressions, e.g., pronouns (it), zeros (…and went down), or descriptive NPs (the pink pentagon). We examined language production in healthy adults, and used anodal transcranial direct current stimulation (tDCS) to test the involvement of the left prefrontal cortex (PFC) in the timing of utterance production and the selection of reference forms in a discourse context. Twenty-two subjects (11 anodal, 11 sham) described fast-paced actions, e.g. The gray oval flashes, then it moves right 2 blocks. We only examined trials in contexts that supported pronoun/zero use. For sham participants, pronouns/zeros increased on trials with longer latencies to initiate the target utterance, and trials where the previous trial was short. We argue that both of these conditions enabled greater message pre-planning and greater discourse connectedness: The strongest predictor of pronoun/zero usage was the presence of a connector word like and or then, which was also tended to occur on trials with longer latencies. For the anodal participants, the latency effect disappeared. PFC stimulation appeared to enable participants to produce utterances with greater discourse connectedness, even while planning incrementally.